DNA methylation may regulate gene expression by restricting
the access of transcription factors. We have
previously demonstrated that GATA-1 regulates the
transcription of the CCR3 gene by dynamically interacting
with both positively and negatively acting GATA
elements of high affinity binding in the proximal promoter
region including exon 1. Exon 1 has three CpG
sites, two of which are positioned at the negatively acting
GATA elements. We hypothesized that the methylation
of these two CpGs sites might preclude GATA-1
binding to the negatively acting GATA elements and,
as a result, increase the availability of GATA-1 to the
positively acting GATA element, thereby contributing
to an increase in GATA-1-mediated transcription of the
gene. To this end, we determined the methylation of the
three CpG sites by bisulfate pyrosequencing in peripheral
blood eosinophils, cord blood (CB)-derived eosinophils,
PBMCs, and cell lines that vary in CCR3 mRNA
expression. Our results demonstrated that methylation
of CpG sites at the negatively acting GATA elements
severely reduced GATA-1 binding and augmented
transcription activity in vitro. In agreement,
methylation of these CpG sites positively correlated
with CCR3 mRNA expression in the primary cells and
cell lines examined. Interestingly, methylation patterns
of these three CpG sites in CB-derived eosinophils
mostly resembled those in peripheral blood
eosinophils. These results suggest that methylation of
CpG sites at the GATA elements in the regulatory regions
fine-tunes CCR3 transcription. |